package x509

import (
    "bytes"
    "crypto"
    "crypto/x509/pkix"
    "errors"
    "fmt"
    "net"
    "net/url"
    "reflect"
    "runtime"
    "strings"
    "time"
    "unicode/utf8"
)

type InvalidReason int

const (
    // NotAuthorizedToSign results when a certificate is signed by another
    // which isn't marked as a CA certificate.
    NotAuthorizedToSign InvalidReason = iota
    // Expired results when a certificate has expired, based on the time
    // given in the VerifyOptions.
    Expired
    // CANotAuthorizedForThisName results when an intermediate or root
    // certificate has a name constraint which doesn't permit a DNS or
    // other name (including IP address) in the leaf certificate.
    CANotAuthorizedForThisName
    // TooManyIntermediates results when a path length constraint is
    // violated.
    TooManyIntermediates
    // IncompatibleUsage results when the certificate's key usage indicates
    // that it may only be used for a different purpose.
    IncompatibleUsage
    // NameMismatch results when the subject name of a parent certificate
    // does not match the issuer name in the child.
    NameMismatch
    // NameConstraintsWithoutSANs is a legacy error and is no longer returned.
    NameConstraintsWithoutSANs
    // UnconstrainedName results when a CA certificate contains permitted
    // name constraints, but leaf certificate contains a name of an
    // unsupported or unconstrained type.
    UnconstrainedName
    // TooManyConstraints results when the number of comparison operations
    // needed to check a certificate exceeds the limit set by
    // VerifyOptions.MaxConstraintComparisions. This limit exists to
    // prevent pathological certificates can consuming excessive amounts of
    // CPU time to verify.
    TooManyConstraints
    // CANotAuthorizedForExtKeyUsage results when an intermediate or root
    // certificate does not permit a requested extended key usage.
    CANotAuthorizedForExtKeyUsage
)

// CertificateInvalidError results when an odd error occurs. Users of this
// library probably want to handle all these errors uniformly.
type CertificateInvalidError struct {
    Cert   *Certificate
    Reason InvalidReason
    Detail string
}

func (e CertificateInvalidError) Error() string {
    switch e.Reason {
    case NotAuthorizedToSign:
        return "x509: certificate is not authorized to sign other certificates"
    case Expired:
        return "x509: certificate has expired or is not yet valid: " + e.Detail
    case CANotAuthorizedForThisName:
        return "x509: a root or intermediate certificate is not authorized to sign for this name: " + e.Detail
    case CANotAuthorizedForExtKeyUsage:
        return "x509: a root or intermediate certificate is not authorized for an extended key usage: " + e.Detail
    case TooManyIntermediates:
        return "x509: too many intermediates for path length constraint"
    case IncompatibleUsage:
        return "x509: certificate specifies an incompatible key usage"
    case NameMismatch:
        return "x509: issuer name does not match subject from issuing certificate"
    case NameConstraintsWithoutSANs:
        return "x509: issuer has name constraints but leaf doesn't have a SAN extension"
    case UnconstrainedName:
        return "x509: issuer has name constraints but leaf contains unknown or unconstrained name: " + e.Detail
    }
    return "x509: unknown error"
}

// HostnameError results when the set of authorized names doesn't match the
// requested name.
type HostnameError struct {
    Certificate *Certificate
    Host        string
}

func (h HostnameError) Error() string {
    c := h.Certificate

    if !c.hasSANExtension() && matchHostnames(c.Subject.CommonName, h.Host) {
        return "x509: certificate relies on legacy Common Name field, use SANs instead"
    }

    var valid string
    if ip := net.ParseIP(h.Host); ip != nil {
        // Trying to validate an IP
        if len(c.IPAddresses) == 0 {
            return "x509: cannot validate certificate for " + h.Host + " because it doesn't contain any IP SANs"
        }
        for _, san := range c.IPAddresses {
            if len(valid) > 0 {
                valid += ", "
            }
            valid += san.String()
        }
    } else {
        valid = strings.Join(c.DNSNames, ", ")
    }

    if len(valid) == 0 {
        return "x509: certificate is not valid for any names, but wanted to match " + h.Host
    }
    return "x509: certificate is valid for " + valid + ", not " + h.Host
}

// UnknownAuthorityError results when the certificate issuer is unknown
type UnknownAuthorityError struct {
    Cert *Certificate
    // hintErr contains an error that may be helpful in determining why an
    // authority wasn't found.
    hintErr error
    // hintCert contains a possible authority certificate that was rejected
    // because of the error in hintErr.
    hintCert *Certificate
}

func (e UnknownAuthorityError) Error() string {
    s := "x509: certificate signed by unknown authority"
    if e.hintErr != nil {
        certName := e.hintCert.Subject.CommonName
        if len(certName) == 0 {
            if len(e.hintCert.Subject.Organization) > 0 {
                certName = e.hintCert.Subject.Organization[0]
            } else {
                certName = "serial:" + e.hintCert.SerialNumber.String()
            }
        }
        s += fmt.Sprintf(" (possibly because of %q while trying to verify candidate authority certificate %q)", e.hintErr, certName)
    }
    return s
}

// SystemRootsError results when we fail to load the system root certificates.
type SystemRootsError struct {
    Err error
}

func (se SystemRootsError) Error() string {
    msg := "x509: failed to load system roots and no roots provided"
    if se.Err != nil {
        return msg + "; " + se.Err.Error()
    }
    return msg
}

func (se SystemRootsError) Unwrap() error { return se.Err }

// errNotParsed is returned when a certificate without ASN.1 contents is
// verified. Platform-specific verification needs the ASN.1 contents.
var errNotParsed = errors.New("x509: missing ASN.1 contents; use ParseCertificate")

// VerifyOptions contains parameters for Certificate.Verify.
type VerifyOptions struct {
    // DNSName, if set, is checked against the leaf certificate with
    // Certificate.VerifyHostname or the platform verifier.
    DNSName string

    // Intermediates is an optional pool of certificates that are not trust
    // anchors, but can be used to form a chain from the leaf certificate to a
    // root certificate.
    Intermediates *CertPool
    // Roots is the set of trusted root certificates the leaf certificate needs
    // to chain up to. If nil, the system roots or the platform verifier are used.
    Roots *CertPool

    // CurrentTime is used to check the validity of all certificates in the
    // chain. If zero, the current time is used.
    CurrentTime time.Time

    // KeyUsages specifies which Extended Key Usage values are acceptable. A
    // chain is accepted if it allows any of the listed values. An empty list
    // means ExtKeyUsageServerAuth. To accept any key usage, include ExtKeyUsageAny.
    KeyUsages []ExtKeyUsage

    // MaxConstraintComparisions is the maximum number of comparisons to
    // perform when checking a given certificate's name constraints. If
    // zero, a sensible default is used. This limit prevents pathological
    // certificates from consuming excessive amounts of CPU time when
    // validating. It does not apply to the platform verifier.
    MaxConstraintComparisions int
}

const (
    leafCertificate = iota
    intermediateCertificate
    rootCertificate
)

// rfc2821Mailbox represents a “mailbox” (which is an email address to most
// people) by breaking it into the “local” (i.e. before the '@') and “domain”
// parts.
type rfc2821Mailbox struct {
    local, domain string
}

// parseRFC2821Mailbox parses an email address into local and domain parts,
// based on the ABNF for a “Mailbox” from RFC 2821. According to RFC 5280,
// Section 4.2.1.6 that's correct for an rfc822Name from a certificate: “The
// format of an rfc822Name is a "Mailbox" as defined in RFC 2821, Section 4.1.2”.
func parseRFC2821Mailbox(in string) (mailbox rfc2821Mailbox, ok bool) {
    if len(in) == 0 {
        return mailbox, false
    }

    localPartBytes := make([]byte, 0, len(in)/2)

    if in[0] == '"' {
        // Quoted-string = DQUOTE *qcontent DQUOTE
        // non-whitespace-control = %d1-8 / %d11 / %d12 / %d14-31 / %d127
        // qcontent = qtext / quoted-pair
        // qtext = non-whitespace-control /
        //         %d33 / %d35-91 / %d93-126
        // quoted-pair = ("\" text) / obs-qp
        // text = %d1-9 / %d11 / %d12 / %d14-127 / obs-text
        //
        // (Names beginning with “obs-” are the obsolete syntax from RFC 2822,
        // Section 4. Since it has been 16 years, we no longer accept that.)
        in = in[1:]
    QuotedString:
        for {
            if len(in) == 0 {
                return mailbox, false
            }
            c := in[0]
            in = in[1:]

            switch {
            case c == '"':
                break QuotedString

            case c == '\\':
                // quoted-pair
                if len(in) == 0 {
                    return mailbox, false
                }
                if in[0] == 11 ||
                    in[0] == 12 ||
                    (1 <= in[0] && in[0] <= 9) ||
                    (14 <= in[0] && in[0] <= 127) {
                    localPartBytes = append(localPartBytes, in[0])
                    in = in[1:]
                } else {
                    return mailbox, false
                }

            case c == 11 ||
                c == 12 ||
                // Space (char 32) is not allowed based on the
                // BNF, but RFC 3696 gives an example that
                // assumes that it is. Several “verified”
                // errata continue to argue about this point.
                // We choose to accept it.
                c == 32 ||
                c == 33 ||
                c == 127 ||
                (1 <= c && c <= 8) ||
                (14 <= c && c <= 31) ||
                (35 <= c && c <= 91) ||
                (93 <= c && c <= 126):
                // qtext
                localPartBytes = append(localPartBytes, c)

            default:
                return mailbox, false
            }
        }
    } else {
        // Atom ("." Atom)*
    NextChar:
        for len(in) > 0 {
            // atext from RFC 2822, Section 3.2.4
            c := in[0]

            switch {
            case c == '\\':
                // Examples given in RFC 3696 suggest that
                // escaped characters can appear outside of a
                // quoted string. Several “verified” errata
                // continue to argue the point. We choose to
                // accept it.
                in = in[1:]
                if len(in) == 0 {
                    return mailbox, false
                }
                fallthrough

            case ('0' <= c && c <= '9') ||
                ('a' <= c && c <= 'z') ||
                ('A' <= c && c <= 'Z') ||
                c == '!' || c == '#' || c == '$' || c == '%' ||
                c == '&' || c == '\'' || c == '*' || c == '+' ||
                c == '-' || c == '/' || c == '=' || c == '?' ||
                c == '^' || c == '_' || c == '`' || c == '{' ||
                c == '|' || c == '}' || c == '~' || c == '.':
                localPartBytes = append(localPartBytes, in[0])
                in = in[1:]

            default:
                break NextChar
            }
        }

        if len(localPartBytes) == 0 {
            return mailbox, false
        }

        // From RFC 3696, Section 3:
        // “period (".") may also appear, but may not be used to start
        // or end the local part, nor may two or more consecutive
        // periods appear.”
        twoDots := []byte{'.', '.'}
        if localPartBytes[0] == '.' ||
            localPartBytes[len(localPartBytes)-1] == '.' ||
            bytes.Contains(localPartBytes, twoDots) {
            return mailbox, false
        }
    }

    if len(in) == 0 || in[0] != '@' {
        return mailbox, false
    }
    in = in[1:]

    // The RFC species a format for domains, but that's known to be
    // violated in practice so we accept that anything after an '@' is the
    // domain part.
    if _, ok := domainToReverseLabels(in); !ok {
        return mailbox, false
    }

    mailbox.local = string(localPartBytes)
    mailbox.domain = in
    return mailbox, true
}

// domainToReverseLabels converts a textual domain name like foo.example.com to
// the list of labels in reverse order, e.g. ["com", "example", "foo"].
func domainToReverseLabels(domain string) (reverseLabels []string, ok bool) {
    for len(domain) > 0 {
        if i := strings.LastIndexByte(domain, '.'); i == -1 {
            reverseLabels = append(reverseLabels, domain)
            domain = ""
        } else {
            reverseLabels = append(reverseLabels, domain[i+1:])
            domain = domain[:i]
        }
    }

    if len(reverseLabels) > 0 && len(reverseLabels[0]) == 0 {
        // An empty label at the end indicates an absolute value.
        return nil, false
    }

    for _, label := range reverseLabels {
        if len(label) == 0 {
            // Empty labels are otherwise invalid.
            return nil, false
        }

        for _, c := range label {
            if c < 33 || c > 126 {
                // Invalid character.
                return nil, false
            }
        }
    }

    return reverseLabels, true
}

func matchEmailConstraint(mailbox rfc2821Mailbox, constraint string) (bool, error) {
    // If the constraint contains an @, then it specifies an exact mailbox
    // name.
    if strings.Contains(constraint, "@") {
        constraintMailbox, ok := parseRFC2821Mailbox(constraint)
        if !ok {
            return false, fmt.Errorf("x509: internal error: cannot parse constraint %q", constraint)
        }
        return mailbox.local == constraintMailbox.local && strings.EqualFold(mailbox.domain, constraintMailbox.domain), nil
    }

    // Otherwise the constraint is like a DNS constraint of the domain part
    // of the mailbox.
    return matchDomainConstraint(mailbox.domain, constraint)
}

func matchURIConstraint(uri *url.URL, constraint string) (bool, error) {
    // From RFC 5280, Section 4.2.1.10:
    // “a uniformResourceIdentifier that does not include an authority
    // component with a host name specified as a fully qualified domain
    // name (e.g., if the URI either does not include an authority
    // component or includes an authority component in which the host name
    // is specified as an IP address), then the application MUST reject the
    // certificate.”

    host := uri.Host
    if len(host) == 0 {
        return false, fmt.Errorf("URI with empty host (%q) cannot be matched against constraints", uri.String())
    }

    if strings.Contains(host, ":") && !strings.HasSuffix(host, "]") {
        var err error
        host, _, err = net.SplitHostPort(uri.Host)
        if err != nil {
            return false, err
        }
    }

    if strings.HasPrefix(host, "[") && strings.HasSuffix(host, "]") ||
        net.ParseIP(host) != nil {
        return false, fmt.Errorf("URI with IP (%q) cannot be matched against constraints", uri.String())
    }

    return matchDomainConstraint(host, constraint)
}

func matchIPConstraint(ip net.IP, constraint *net.IPNet) (bool, error) {
    if len(ip) != len(constraint.IP) {
        return false, nil
    }

    for i := range ip {
        if mask := constraint.Mask[i]; ip[i]&mask != constraint.IP[i]&mask {
            return false, nil
        }
    }

    return true, nil
}

func matchDomainConstraint(domain, constraint string) (bool, error) {
    // The meaning of zero length constraints is not specified, but this
    // code follows NSS and accepts them as matching everything.
    if len(constraint) == 0 {
        return true, nil
    }

    domainLabels, ok := domainToReverseLabels(domain)
    if !ok {
        return false, fmt.Errorf("x509: internal error: cannot parse domain %q", domain)
    }

    // RFC 5280 says that a leading period in a domain name means that at
    // least one label must be prepended, but only for URI and email
    // constraints, not DNS constraints. The code also supports that
    // behaviour for DNS constraints.

    mustHaveSubdomains := false
    if constraint[0] == '.' {
        mustHaveSubdomains = true
        constraint = constraint[1:]
    }

    constraintLabels, ok := domainToReverseLabels(constraint)
    if !ok {
        return false, fmt.Errorf("x509: internal error: cannot parse domain %q", constraint)
    }

    if len(domainLabels) < len(constraintLabels) ||
        (mustHaveSubdomains && len(domainLabels) == len(constraintLabels)) {
        return false, nil
    }

    for i, constraintLabel := range constraintLabels {
        if !strings.EqualFold(constraintLabel, domainLabels[i]) {
            return false, nil
        }
    }

    return true, nil
}

// checkNameConstraints checks that c permits a child certificate to claim the
// given name, of type nameType. The argument parsedName contains the parsed
// form of name, suitable for passing to the match function. The total number
// of comparisons is tracked in the given count and should not exceed the given
// limit.
func (c *Certificate) checkNameConstraints(count *int,
    maxConstraintComparisons int,
    nameType string,
    name string,
    parsedName any,
    match func(parsedName, constraint any) (match bool, err error),
    permitted, excluded any) error {

    excludedValue := reflect.ValueOf(excluded)

    *count += excludedValue.Len()
    if *count > maxConstraintComparisons {
        return CertificateInvalidError{c, TooManyConstraints, ""}
    }

    for i := 0; i < excludedValue.Len(); i++ {
        constraint := excludedValue.Index(i).Interface()
        match, err := match(parsedName, constraint)
        if err != nil {
            return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()}
        }

        if match {
            return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is excluded by constraint %q", nameType, name, constraint)}
        }
    }

    permittedValue := reflect.ValueOf(permitted)

    *count += permittedValue.Len()
    if *count > maxConstraintComparisons {
        return CertificateInvalidError{c, TooManyConstraints, ""}
    }

    ok := true
    for i := 0; i < permittedValue.Len(); i++ {
        constraint := permittedValue.Index(i).Interface()

        var err error
        if ok, err = match(parsedName, constraint); err != nil {
            return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()}
        }

        if ok {
            break
        }
    }

    if !ok {
        return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is not permitted by any constraint", nameType, name)}
    }

    return nil
}

// isValid performs validity checks on c given that it is a candidate to append
// to the chain in currentChain.
func (c *Certificate) isValid(certType int, currentChain []*Certificate, opts *VerifyOptions) error {
    if len(c.UnhandledCriticalExtensions) > 0 {
        return UnhandledCriticalExtension{}
    }

    if len(currentChain) > 0 {
        child := currentChain[len(currentChain)-1]
        if !bytes.Equal(child.RawIssuer, c.RawSubject) {
            return CertificateInvalidError{c, NameMismatch, ""}
        }
    }

    now := opts.CurrentTime
    if now.IsZero() {
        now = time.Now()
    }
    if now.Before(c.NotBefore) {
        return CertificateInvalidError{
            Cert:   c,
            Reason: Expired,
            Detail: fmt.Sprintf("current time %s is before %s", now.Format(time.RFC3339), c.NotBefore.Format(time.RFC3339)),
        }
    } else if now.After(c.NotAfter) {
        return CertificateInvalidError{
            Cert:   c,
            Reason: Expired,
            Detail: fmt.Sprintf("current time %s is after %s", now.Format(time.RFC3339), c.NotAfter.Format(time.RFC3339)),
        }
    }

    maxConstraintComparisons := opts.MaxConstraintComparisions
    if maxConstraintComparisons == 0 {
        maxConstraintComparisons = 250000
    }
    comparisonCount := 0

    if certType == intermediateCertificate || certType == rootCertificate {
        if len(currentChain) == 0 {
            return errors.New("x509: internal error: empty chain when appending CA cert")
        }
    }

    if (certType == intermediateCertificate || certType == rootCertificate) &&
        c.hasNameConstraints() {
        toCheck := []*Certificate{}
        for _, c := range currentChain {
            if c.hasSANExtension() {
                toCheck = append(toCheck, c)
            }
        }
        for _, sanCert := range toCheck {
            err := forEachSAN(sanCert.getSANExtension(), func(tag int, data []byte) error {
                switch tag {
                case nameTypeEmail:
                    name := string(data)
                    mailbox, ok := parseRFC2821Mailbox(name)
                    if !ok {
                        return fmt.Errorf("x509: cannot parse rfc822Name %q", mailbox)
                    }

                    if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "email address", name, mailbox,
                        func(parsedName, constraint any) (bool, error) {
                            return matchEmailConstraint(parsedName.(rfc2821Mailbox), constraint.(string))
                        }, c.PermittedEmailAddresses, c.ExcludedEmailAddresses); err != nil {
                        return err
                    }

                case nameTypeDNS:
                    name := string(data)
                    if _, ok := domainToReverseLabels(name); !ok {
                        return fmt.Errorf("x509: cannot parse dnsName %q", name)
                    }

                    if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "DNS name", name, name,
                        func(parsedName, constraint any) (bool, error) {
                            return matchDomainConstraint(parsedName.(string), constraint.(string))
                        }, c.PermittedDNSDomains, c.ExcludedDNSDomains); err != nil {
                        return err
                    }

                case nameTypeURI:
                    name := string(data)
                    uri, err := url.Parse(name)
                    if err != nil {
                        return fmt.Errorf("x509: internal error: URI SAN %q failed to parse", name)
                    }

                    if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "URI", name, uri,
                        func(parsedName, constraint any) (bool, error) {
                            return matchURIConstraint(parsedName.(*url.URL), constraint.(string))
                        }, c.PermittedURIDomains, c.ExcludedURIDomains); err != nil {
                        return err
                    }

                case nameTypeIP:
                    ip := net.IP(data)
                    if l := len(ip); l != net.IPv4len && l != net.IPv6len {
                        return fmt.Errorf("x509: internal error: IP SAN %x failed to parse", data)
                    }

                    if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "IP address", ip.String(), ip,
                        func(parsedName, constraint any) (bool, error) {
                            return matchIPConstraint(parsedName.(net.IP), constraint.(*net.IPNet))
                        }, c.PermittedIPRanges, c.ExcludedIPRanges); err != nil {
                        return err
                    }

                default:
                    // Unknown SAN types are ignored.
                }

                return nil
            })

            if err != nil {
                return err
            }
        }
    }

    // KeyUsage status flags are ignored. From Engineering Security, Peter
    // Gutmann: A European government CA marked its signing certificates as
    // being valid for encryption only, but no-one noticed. Another
    // European CA marked its signature keys as not being valid for
    // signatures. A different CA marked its own trusted root certificate
    // as being invalid for certificate signing. Another national CA
    // distributed a certificate to be used to encrypt data for the
    // country’s tax authority that was marked as only being usable for
    // digital signatures but not for encryption. Yet another CA reversed
    // the order of the bit flags in the keyUsage due to confusion over
    // encoding endianness, essentially setting a random keyUsage in
    // certificates that it issued. Another CA created a self-invalidating
    // certificate by adding a certificate policy statement stipulating
    // that the certificate had to be used strictly as specified in the
    // keyUsage, and a keyUsage containing a flag indicating that the RSA
    // encryption key could only be used for Diffie-Hellman key agreement.

    if certType == intermediateCertificate && (!c.BasicConstraintsValid || !c.IsCA) {
        return CertificateInvalidError{c, NotAuthorizedToSign, ""}
    }

    if c.BasicConstraintsValid && c.MaxPathLen >= 0 {
        numIntermediates := len(currentChain) - 1
        if numIntermediates > c.MaxPathLen {
            return CertificateInvalidError{c, TooManyIntermediates, ""}
        }
    }

    if !boringAllowCert(c) {
        // IncompatibleUsage is not quite right here,
        // but it's also the "no chains found" error
        // and is close enough.
        return CertificateInvalidError{c, IncompatibleUsage, ""}
    }

    return nil
}

// Verify attempts to verify c by building one or more chains from c to a
// certificate in opts.Roots, using certificates in opts.Intermediates if
// needed. If successful, it returns one or more chains where the first
// element of the chain is c and the last element is from opts.Roots.
//
// If opts.Roots is nil, the platform verifier might be used, and
// verification details might differ from what is described below. If system
// roots are unavailable the returned error will be of type SystemRootsError.
//
// Name constraints in the intermediates will be applied to all names claimed
// in the chain, not just opts.DNSName. Thus it is invalid for a leaf to claim
// example.com if an intermediate doesn't permit it, even if example.com is not
// the name being validated. Note that DirectoryName constraints are not
// supported.
//
// Name constraint validation follows the rules from RFC 5280, with the
// addition that DNS name constraints may use the leading period format
// defined for emails and URIs. When a constraint has a leading period
// it indicates that at least one additional label must be prepended to
// the constrained name to be considered valid.
//
// Extended Key Usage values are enforced nested down a chain, so an intermediate
// or root that enumerates EKUs prevents a leaf from asserting an EKU not in that
// list. (While this is not specified, it is common practice in order to limit
// the types of certificates a CA can issue.)
//
// Certificates that use SHA1WithRSA and ECDSAWithSHA1 signatures are not supported,
// and will not be used to build chains.
//
// Certificates other than c in the returned chains should not be modified.
//
// WARNING: this function doesn't do any revocation checking.
func (c *Certificate) Verify(opts VerifyOptions) (chains [][]*Certificate, err error) {
    // Platform-specific verification needs the ASN.1 contents so
    // this makes the behavior consistent across platforms.
    if len(c.Raw) == 0 {
        return nil, errNotParsed
    }

    for i := 0; i < opts.Intermediates.len(); i++ {
        c, _, err := opts.Intermediates.cert(i)
        if err != nil {
            return nil, fmt.Errorf("crypto/x509: error fetching intermediate: %w", err)
        }

        if len(c.Raw) == 0 {
            return nil, errNotParsed
        }
    }

    // Use platform verifiers, where available, if Roots is from SystemCertPool.
    if runtime.GOOS == "windows" || runtime.GOOS == "darwin" || runtime.GOOS == "ios" {
        // Don't use the system verifier if the system pool was replaced with a non-system pool,
        // i.e. if SetFallbackRoots was called with x509usefallbackroots=1.
        systemPool := systemRootsPool()
        if opts.Roots == nil && (systemPool == nil || systemPool.systemPool) {
            return c.systemVerify(&opts)
        }
        if opts.Roots != nil && opts.Roots.systemPool {
            platformChains, err := c.systemVerify(&opts)
            // If the platform verifier succeeded, or there are no additional
            // roots, return the platform verifier result. Otherwise, continue
            // with the Go verifier.
            if err == nil || opts.Roots.len() == 0 {
                return platformChains, err
            }
        }
    }

    if opts.Roots == nil {
        opts.Roots = systemRootsPool()
        if opts.Roots == nil {
            return nil, SystemRootsError{systemRootsErr}
        }
    }

    err = c.isValid(leafCertificate, nil, &opts)
    if err != nil {
        return
    }

    if len(opts.DNSName) > 0 {
        err = c.VerifyHostname(opts.DNSName)
        if err != nil {
            return
        }
    }

    var candidateChains [][]*Certificate
    if opts.Roots.contains(c) {
        candidateChains = [][]*Certificate{{c}}
    } else {
        candidateChains, err = c.buildChains([]*Certificate{c}, nil, &opts)
        if err != nil {
            return nil, err
        }
    }

    if len(opts.KeyUsages) == 0 {
        opts.KeyUsages = []ExtKeyUsage{ExtKeyUsageServerAuth}
    }

    for _, eku := range opts.KeyUsages {
        if eku == ExtKeyUsageAny {
            // If any key usage is acceptable, no need to check the chain for
            // key usages.
            return candidateChains, nil
        }
    }

    chains = make([][]*Certificate, 0, len(candidateChains))
    for _, candidate := range candidateChains {
        if checkChainForKeyUsage(candidate, opts.KeyUsages) {
            chains = append(chains, candidate)
        }
    }

    if len(chains) == 0 {
        return nil, CertificateInvalidError{c, IncompatibleUsage, ""}
    }

    return chains, nil
}

func appendToFreshChain(chain []*Certificate, cert *Certificate) []*Certificate {
    n := make([]*Certificate, len(chain)+1)
    copy(n, chain)
    n[len(chain)] = cert
    return n
}

// alreadyInChain checks whether a candidate certificate is present in a chain.
// Rather than doing a direct byte for byte equivalency check, we check if the
// subject, public key, and SAN, if present, are equal. This prevents loops that
// are created by mutual cross-signatures, or other cross-signature bridge
// oddities.
func alreadyInChain(candidate *Certificate, chain []*Certificate) bool {
    type pubKeyEqual interface {
        Equal(crypto.PublicKey) bool
    }

    var candidateSAN *pkix.Extension
    for _, ext := range candidate.Extensions {
        if ext.Id.Equal(oidExtensionSubjectAltName) {
            candidateSAN = &ext
            break
        }
    }

    for _, cert := range chain {
        if !bytes.Equal(candidate.RawSubject, cert.RawSubject) {
            continue
        }
        if !candidate.PublicKey.(pubKeyEqual).Equal(cert.PublicKey) {
            continue
        }
        var certSAN *pkix.Extension
        for _, ext := range cert.Extensions {
            if ext.Id.Equal(oidExtensionSubjectAltName) {
                certSAN = &ext
                break
            }
        }
        if candidateSAN == nil && certSAN == nil {
            return true
        } else if candidateSAN == nil || certSAN == nil {
            return false
        }
        if bytes.Equal(candidateSAN.Value, certSAN.Value) {
            return true
        }
    }
    return false
}

// maxChainSignatureChecks is the maximum number of CheckSignatureFrom calls
// that an invocation of buildChains will (transitively) make. Most chains are
// less than 15 certificates long, so this leaves space for multiple chains and
// for failed checks due to different intermediates having the same Subject.
const maxChainSignatureChecks = 100

func (c *Certificate) buildChains(currentChain []*Certificate, sigChecks *int, opts *VerifyOptions) (chains [][]*Certificate, err error) {
    var (
        hintErr  error
        hintCert *Certificate
    )

    considerCandidate := func(certType int, candidate *Certificate) {
        if alreadyInChain(candidate, currentChain) {
            return
        }

        if sigChecks == nil {
            sigChecks = new(int)
        }
        *sigChecks++
        if *sigChecks > maxChainSignatureChecks {
            err = errors.New("x509: signature check attempts limit reached while verifying certificate chain")
            return
        }

        if err := c.CheckSignatureFrom(candidate); err != nil {
            if hintErr == nil {
                hintErr = err
                hintCert = candidate
            }
            return
        }

        err = candidate.isValid(certType, currentChain, opts)
        if err != nil {
            if hintErr == nil {
                hintErr = err
                hintCert = candidate
            }
            return
        }

        switch certType {
        case rootCertificate:
            chains = append(chains, appendToFreshChain(currentChain, candidate))
        case intermediateCertificate:
            var childChains [][]*Certificate
            childChains, err = candidate.buildChains(appendToFreshChain(currentChain, candidate), sigChecks, opts)
            chains = append(chains, childChains...)
        }
    }

    for _, root := range opts.Roots.findPotentialParents(c) {
        considerCandidate(rootCertificate, root.cert)
    }
    for _, intermediate := range opts.Intermediates.findPotentialParents(c) {
        considerCandidate(intermediateCertificate, intermediate.cert)
    }

    if len(chains) > 0 {
        err = nil
    }
    if len(chains) == 0 && err == nil {
        err = UnknownAuthorityError{c, hintErr, hintCert}
    }

    return
}

func validHostnamePattern(host string) bool { return validHostname(host, true) }
func validHostnameInput(host string) bool   { return validHostname(host, false) }

// validHostname reports whether host is a valid hostname that can be matched or
// matched against according to RFC 6125 2.2, with some leniency to accommodate
// legacy values.
func validHostname(host string, isPattern bool) bool {
    if !isPattern {
        host = strings.TrimSuffix(host, ".")
    }
    if len(host) == 0 {
        return false
    }

    for i, part := range strings.Split(host, ".") {
        if part == "" {
            // Empty label.
            return false
        }
        if isPattern && i == 0 && part == "*" {
            // Only allow full left-most wildcards, as those are the only ones
            // we match, and matching literal '*' characters is probably never
            // the expected behavior.
            continue
        }
        for j, c := range part {
            if 'a' <= c && c <= 'z' {
                continue
            }
            if '0' <= c && c <= '9' {
                continue
            }
            if 'A' <= c && c <= 'Z' {
                continue
            }
            if c == '-' && j != 0 {
                continue
            }
            if c == '_' {
                // Not a valid character in hostnames, but commonly
                // found in deployments outside the WebPKI.
                continue
            }
            return false
        }
    }

    return true
}

func matchExactly(hostA, hostB string) bool {
    if hostA == "" || hostA == "." || hostB == "" || hostB == "." {
        return false
    }
    return toLowerCaseASCII(hostA) == toLowerCaseASCII(hostB)
}

func matchHostnames(pattern, host string) bool {
    pattern = toLowerCaseASCII(pattern)
    host = toLowerCaseASCII(strings.TrimSuffix(host, "."))

    if len(pattern) == 0 || len(host) == 0 {
        return false
    }

    patternParts := strings.Split(pattern, ".")
    hostParts := strings.Split(host, ".")

    if len(patternParts) != len(hostParts) {
        return false
    }

    for i, patternPart := range patternParts {
        if i == 0 && patternPart == "*" {
            continue
        }
        if patternPart != hostParts[i] {
            return false
        }
    }

    return true
}

// toLowerCaseASCII returns a lower-case version of in. See RFC 6125 6.4.1. We use
// an explicitly ASCII function to avoid any sharp corners resulting from
// performing Unicode operations on DNS labels.
func toLowerCaseASCII(in string) string {
    // If the string is already lower-case then there's nothing to do.
    isAlreadyLowerCase := true
    for _, c := range in {
        if c == utf8.RuneError {
            // If we get a UTF-8 error then there might be
            // upper-case ASCII bytes in the invalid sequence.
            isAlreadyLowerCase = false
            break
        }
        if 'A' <= c && c <= 'Z' {
            isAlreadyLowerCase = false
            break
        }
    }

    if isAlreadyLowerCase {
        return in
    }

    out := []byte(in)
    for i, c := range out {
        if 'A' <= c && c <= 'Z' {
            out[i] += 'a' - 'A'
        }
    }
    return string(out)
}

// VerifyHostname returns nil if c is a valid certificate for the named host.
// Otherwise it returns an error describing the mismatch.
//
// IP addresses can be optionally enclosed in square brackets and are checked
// against the IPAddresses field. Other names are checked case insensitively
// against the DNSNames field. If the names are valid hostnames, the certificate
// fields can have a wildcard as the complete left-most label (e.g. *.example.com).
//
// Note that the legacy Common Name field is ignored.
func (c *Certificate) VerifyHostname(h string) error {
    // IP addresses may be written in [ ].
    candidateIP := h
    if len(h) >= 3 && h[0] == '[' && h[len(h)-1] == ']' {
        candidateIP = h[1 : len(h)-1]
    }
    if ip := net.ParseIP(candidateIP); ip != nil {
        // We only match IP addresses against IP SANs.
        // See RFC 6125, Appendix B.2.
        for _, candidate := range c.IPAddresses {
            if ip.Equal(candidate) {
                return nil
            }
        }
        return HostnameError{c, candidateIP}
    }

    candidateName := toLowerCaseASCII(h) // Save allocations inside the loop.
    validCandidateName := validHostnameInput(candidateName)

    for _, match := range c.DNSNames {
        // Ideally, we'd only match valid hostnames according to RFC 6125 like
        // browsers (more or less) do, but in practice Go is used in a wider
        // array of contexts and can't even assume DNS resolution. Instead,
        // always allow perfect matches, and only apply wildcard and trailing
        // dot processing to valid hostnames.
        if validCandidateName && validHostnamePattern(match) {
            if matchHostnames(match, candidateName) {
                return nil
            }
        } else {
            if matchExactly(match, candidateName) {
                return nil
            }
        }
    }

    return HostnameError{c, h}
}

func checkChainForKeyUsage(chain []*Certificate, keyUsages []ExtKeyUsage) bool {
    usages := make([]ExtKeyUsage, len(keyUsages))
    copy(usages, keyUsages)

    if len(chain) == 0 {
        return false
    }

    usagesRemaining := len(usages)

    // We walk down the list and cross out any usages that aren't supported
    // by each certificate. If we cross out all the usages, then the chain
    // is unacceptable.

NextCert:
    for i := len(chain) - 1; i >= 0; i-- {
        cert := chain[i]
        if len(cert.ExtKeyUsage) == 0 && len(cert.UnknownExtKeyUsage) == 0 {
            // The certificate doesn't have any extended key usage specified.
            continue
        }

        for _, usage := range cert.ExtKeyUsage {
            if usage == ExtKeyUsageAny {
                // The certificate is explicitly good for any usage.
                continue NextCert
            }
        }

        const invalidUsage ExtKeyUsage = -1

    NextRequestedUsage:
        for i, requestedUsage := range usages {
            if requestedUsage == invalidUsage {
                continue
            }

            for _, usage := range cert.ExtKeyUsage {
                if requestedUsage == usage {
                    continue NextRequestedUsage
                }
            }

            usages[i] = invalidUsage
            usagesRemaining--
            if usagesRemaining == 0 {
                return false
            }
        }
    }

    return true
}
